This invention concerns sliding bearings for chocks in rolling mill stands with crossed displacement of the rolls under load.
The sliding bearings are applied in cooperation with the chocks of the four-high rolling mill stands to produce hot rolled sheet and/or large plate which include the crossed displacement of the rolls also during the hot rolling cycle.
The state of the art covers four-high rolling mill stands which comprise two opposed working rolls associated with relative back-up rolls with the function of limiting the bending of the working rolls during the rolling step.
Moreover the state of the art also covers rolling techniques which include the reciprocal pair crossing of both pairs of rolls or at least the crossed displacement of the working rolls alone.
These techniques make it possible to control more accurately the profile of the rolled product and therefore more generally to obtain products of a higher quality.
At the present time the pair crossing movements are carried out during the resting stage between the rolling of two successive slabs; this is necessary because of the considerable thrust forces transmitted by the rolls during the passage of the rolled product which make this displacement practically impossible during the rolling step.
These thrust forces generate friction between the chocks of the upper and lower back-up rolls and the respective organs, such as millscrews, hydraulic actuator capsules, spacers, etc., which discharge the rolling force onto the housing of the rolling mill stand.
This friction contrasts the pair crossing movement.
The introduction of continuous rolling of sheet or large plate, with welding of the ends of the individual slabs, has highlighted this problem of the pair crossing of rolls, which in this case must necessarily take place also during the processing step.
To carry out pair crossing in rolling mill stands such as those known to the state of the art is in fact extremely difficult and inaccurate because of the above-mentioned friction which contrasts the crossing movements; this causes disfunctions and/or damage in the rolling assembly, it causes products of an inferior quality to be obtained, wear in the components which are in reciprocal contact, high powers in play and a whole series of other disadvantages.
Various solutions have therefore been proposed to solve the problem of moving the rolls under load with respect to the relative chocks, but these solutions have not been able to solve the problem efficiently.
JP 57-193211 teaches to use sliding bearings suitable to reduce the friction between the supporting chocks of the back-up rolls and the corresponding equalizer beams on which the adjustment means of the stand act.
The equalizer beams make the structure of the stand heavy, and also make the conventional operations of adjusting the rolls and transmitting the rolling load less precise.
The sliding bearings consist of a series of cylindrical rollers arranged parallel to each other and separated in such a manner as to cover substantially the entire width of the relative chock.
This parallel arrangement of the cylindrical rollers, and their cylindrical shape itself, causes a high level of rubbing on the horizontal plane, both between the cylindrical rollers and the chock and also between the cylindrical rollers and the stationary housing; on the one hand this makes the pair crossing adjustment very imprecise and on the other hand it requires high forces of thrust to be used. Moreover the cylindrical rollers are subjected to anomalous stress, with localised and disuniform overloads.
GB-A-2141959 describes friction-reducing means interposed between the chock and the housing and not between the chock and the means to adjust the rolls.
The friction-reducing means can include, in the various solutions proposed, limiting plates inside which a fluid is made to circulate, a series of cylindrical rollers arranged parallel to each other on the width of the relative chock and a series of pads made of high resistance elastic material, for example rubber or similar.
In the first case, the plates to limit the fluid cause problems if the rolling stand includes systems to adjust the rolls and to transmit the load placed between the housing and the chock.
Moreover, they create problems of sliding friction and therefore of wear caused by rubbing between the parts in reciprocal movement.
The system with parallel cylindrical rollers has the same problems as those mentioned above with regard to JP'211, while the system with elastic pads does not guarantee a sufficient reduction in the friction, given the extremely high forces of thrust which act between the housing and the chock when the rolls are under load.
JP 06-269812 does not refer to a four-high stand and includes friction-reducing means between the supporting chock of the working rolls and the stationary housing. These means consist of two plates arranged in contact with each other defining small chambers into which fluid under pressure is fed.
The surface of the parts in contact is very large, and this causes a minimum reduction of the friction, and premature wear; moreover, a great force of thrust is required due to the sliding friction which develops between the two parts in reciprocal movement.
The Research Disclosure n.degree.. 293, September 1988, simply describes the introduction of lubricating fluid into pads located between the hydraulic capsules and the relative chocks, but this solution does not solve any of the above-mentioned problems.
JP 04-55004 describes the use of cylindrical bearings consisting of a plurality of small rollers of very reduced diameter arranged radially with their axis lying on the radius of the circumference where the centre is the point of rotation of the chock.
This solution, although it improves on the solution with the cylindrical rollers arranged parallel, does not completely solve the problems which derive from using small cylindrical rollers which in any case cause horizontal rubbing of the parts in reciprocal movement precisely because of the cylindrical shape of the friction-reducing rollers.
Moreover, this solution involves complex construction, assembly and adjustment, and also keeps wide areas without rollers, with a high concentration of loads, which concentration is accentuated by the small size of the rollers themselves.
Moreover, this document also proposes using an equalizer plate placed between the chock and the housing.
For this reason, it does not solve the problems of decreased accuracy of the crossover movements, the need to use extremely high displacement forces, and the premature wear of the parts in reciprocal contact.